Method for offshore loading of a tanker and construction of said tanker

ABSTRACT

A method and a vessel for offshore oil loading, wherein the vessel comprises a number of center tanks (3) arranged in the longitudinal direction of the vessel, and a number of wing tanks (5, 6) located in pairs on either side of respective center tanks, a loading line (19) being connected to the center tanks (3) via respective valves (20), and adjacent center tanks (3) being connected to each other via bulkhead valves (18). The method comprises the steps of a) firstly effecting filling of a desired number of the center tanks (3), these being substantially narrower and having a correspondingly smaller volume than the wing tanks (5, 6), b) thereafter opening bulkhead valves (17) between a center tank (3) and an appurtenant wing tank pair (5, 6), and sluicing out oil from the center tanks (3) into the two wing tanks (5, 6) by means of the static pressure from the center tanks, the bulkhead valves (18) between the center tanks (3) being open, so that the wing tank pair (5, 6) is filled as quickly as possible by suitable operation of the bulkhead valves (18) between the center tanks (3) and of the valves (20) of the loading line (19) to the center tanks (3), c) closing the bulkhead valves (17) of the two wing tanks (5, 6) when these and the appurtenant center tank (3) are filled to a desired level, and d) repeating the filling process according to the steps b) and c) successively for additional wing tank pairs (5, 6) after that all center tanks (3) have been filled again.

The invention relates to a method and a vessel for offshore oil loading,wherein the vessel comprises a number of center tanks arranged in thelongitudinal direction of the vessel, and a number of wing tanks locatedin pairs on either side of respective center tanks, a loading line beingconnected to the center tanks via respective valves, and adjacent centertanks being connected to each other via bulkhead valves.

In connection with offshore oil loading a part of the load will be lostto the atmosphere because of de-gassing from the liquid surface. As anexample, in a typical buoy loading in the North Sea, there may be lostapproximately 200 tons per loading of tankers of about 120 000 tons. Therelease of gas mainly is due to splashing and stirring of crude oil inthe loading tanks, and occurring pressure drop in the hydrocarbon loadduring the handling thereof.

Measurements have shown that the extent of the degassing is proportionalto the time before the tanks are full, and the period of the conditionof standing waves (resonance) in the tanks. In addition, loads on thetank structure will be increasing with the period of standing waves. Itis therefore essential to fill the individual tanks as quickly aspossible, both to reduce the de-gassing period and therewith the totalspill of gases, and to reduce loads on the tank structures duringloading.

Thus, the object of the invention is to provide a method and a vesselresulting in a substantial reduction of the degassing during loading,and also in a substantial reduction of loads on the tank structures.

It is also an object of the invention to provide a vessel having a tankarrangement which has technical and economic advantages, especially withdirect loading of tankers on the field.

For the achievement of the above-mentioned objects there is provided amethod of the introductorily stated type which, according to theinvention, is characterized by the steps of

a) firstly effecting filling of a desired number of the center tanks,these being substantially narrower and having a correspondingly smallervolume than the wing tanks,

b) thereafter opening bulkhead valves between a center tank and anappurtenant wing tank pair and sluicing out oil from the center tanksinto the two wing tanks by means of the static pressure from the centertanks, the bulkhead valves between the center tanks being open, so thatthe wing tank pair is filled as quickly as possible by suitableoperation of the bulkhead valves between the center tanks and of thevalves of the loading line to the center tanks,

c) closing the bulkhead valves to the two wing tanks when these and theappurtenant center tank are filled to a desired level, and

d) repeating the filling process according to the steps b) and c)successively for additional wing tank pairs after that all center tankshave been filled again.

According to the invention there is also provided a vessel of theintroductorily stated type which, according to the invention, ischaracterized in that the center tanks have a width which is less thanhalf the width of the wing tanks, that all tank triplets consisting of acenter tank and an appurtenant wing tank pair are connected to eachother via bulkhead valves, and that the loading line extends through thecenter tanks, all valves with appurtenant drive units being arrangedcentrally, in connection with the center tank area.

The invention will be further described below in connection withexemplary embodiments with reference to the drawings, wherein

FIGS. 1, 2 and 3 show a side view, a plan view and a cross-section,respectively, of a vessel having a tank arrangement according to theinvention;

FIGS. 4 and 5 show a side view and plan view, respectively, of thevessel in FIGS. 1-3, and show the principal set-up of conduits andvalves;

FIGS. 6 and 7 show cross-sections of two different bottom arrangements;and

FIGS. 8, 9 and 10 are a side view, a plan view and a cross-section,respectively, showing a separate emergency loading or "stripping"arrangement which is also installed on the vessel.

In the drawings there is shown a vessel 1 in the form of a tankerwherein a pair of longitudinal bulkheads 2 are arranged so that the tankarea of the vessel is divided into a narrow center tank section and apair of relatively wide wing tank sections. In the illustratedembodiment six tank triplets I-VI are arranged in the longitudinaldirection of the ship, each of the triplets consisting of a center tank3 and a pair of wing tanks 5, 6 which are located on either side of thecenter tank. The center tanks 3 have a width which is about one third ofthe width of the wing tanks, so that if the width of the ship is b inthe loading tank area, the center tank has a width of about 1/7 b,whereas the wing tanks have a width of about 3/7 b.

As shown, in the center tanks 3 there are arranged rolling bulkheads orbaffle plates 4 running from side to side in the tanks and extending upto about half the tank height. These have the function to preventformation of longitudinally extending, standing waves in the oil cargo.If necessary, such bulkheads may also be arranged in the wing tanks.

As shown, ballast tanks 7 are arranged around the loading tanks by meansof double hulls, and also the bottom of the vessel is executed as adouble bottom, for the formation of bottom ballast tanks 8. The ballasttanks may be filled with ballast for stabilization purposes, filled oremptied selectively for trimming, etc.

As a dimensioning example, the center tanks may have a width of about 6m, the wing tanks a width of about 18 m, and the ballast tanks on eachside a width of about 2 m, with a total ship width of about 46 m.

The vessel 1 is built as a buoy loading tanker, and for this purpose isprovided with a submerged receiving space 9 in the bow portion of thevessel, for the reception of an adapted underwater buoy (not shown), andwith a vertical shaft 10 extending between the receiving space and thedeck of the vessel. A vessel for operation with such a buoy is describedin the international patent application No. PCT/N092/00055.

The vessel may, however, also be an offshore storage tanker or aproduction vessel, oil loading according to the present inventionequally well being able to take place on such vessels.

The receiving space 9 and the loading equipment 11 for the topical buoyin the illustrated embodiment are arranged behind a forward collisionbulkhead 12. As an alternative, the receiving space, the loadingequipment and said shaft may be arranged at least partly in front of thecollision bulkhead.

The vessel is shown to be provided with three bow thrusters 13 and twostern thrusters 14.

Behind the rearmost tank triplet VI there is arranged a pump room 15 forunloading pumps with an appurtenant pipe and valve arrangement, as shownin FIGS. 4 and 5. On each side of the pump room there is a slop or wastewater tank 16.

A typical ship cross-section is shown in FIG. 3. As shown, the wingtanks 5, 6 have a tilted bottom inwards towards the center tank 3, tosecure a good flow to the center tank section when unloading. In orderto obtain an additionally improved inflow from the wing tanks, thebottom of the center tanks may be located somewhat lower than theadjacent bottom area of the wing tanks, as shown in FIG. 7.

The conduit and valve arrangement for loading and unloading is shown inFIGS. 4 and 5.

In each tank triplet I-VI, the center tank 3 is connected to the pair ofwing tanks 5, 6 by means of forward and rearward bulkhead valves 17.Further, all center tanks 3 are connected to each other through bulkheadvalves 18. The bulkhead valves suitably may consist of "lying" flapvalves, in order to have--in a simple manner--the valve opening placedas close to the bottom of the tank as possible. A typical size of thebulkhead valves is 500 mm in diameter.

A loading line or loading conduit 19 extends through all the centertanks 3 and is connected to the center tanks via respective loading linevalves 20, so that the loading conduit or loading line is able to serveall the center tanks singly or collectively. On the loading line 19there is also arranged pipe valves 38 in each of the center tanks 3. Asshown, the loading line 19 extends as a bottom line through the centertanks, the loading line at its forward end being coupled directly to theloading equipment 11 for connection to the topical loading buoy in thereceiving space 9, and at its rearward end being coupled to pumping andvalve equipment in the pump room 15. As a result of the fact that theloading line along its entire length is located at the same level,underpressure during loading is avoided, and thereby transition from oilto gas in the pipe system, in the way it is typical in conventionaloffshore buoy loading, is prevented.

As shown in FIG. 4, the loading line 19 is coupled to the loadingequipment 11 via at least one shut-off or stop valve 21. The loadingline here is also coupled to a line or conduit 22 for traditional buoyloading.

In the pump room 15, the loading line 19 is coupled to a pump 23,through suitable valves. The pump is used in unloading of the tanks andis connected to an external unloading line 24. As shown, there is alsoarranged a so-called "cross-over" or transition connection 25, for theprovision of an alternative pumping path, with a view to flexibility.

In addition to the loading line 19, the pipe arrangement in theillustrated embodiment comprises a pair of additional main pipes, morespecifically a second conduit 26 which extends from the pump room 15forwards to the center tank in the first tank triplet I and is connectedto all center tanks via respective line valves 27, and a third conduit28 which extends from the pump room 15 forwards to the center tank inthe fifth tank triplet V and is connected to the center tanks in thetank triplets V and VI via line valves 29. These additional conduits 26and 28 are connected to respective pumps 30 and 31 which are alsocoupled to external unloading lines. As shown, the conduits 19, 26 and28 in the pump room are connected to each other and connected to thewaste water tanks 16 via a suitable valve arrangement. The arrangementwith the conduit 26 implies that there may also be pumped from thecenter tanks 3 into the tank triplet which is to be filled, as furtherdescribed later.

For achieving a tripartite segregation of the tank area, double bulkheadvalves 32 and 33 are arranged between the center tanks in the tanktriplets II and III and the tank triplets IV and V, respectively.Moreover, the loading line 19 is provided with double valves in thecenter tanks in the tank triplets III-VI, and the conduit 26 is alsoprovided with double valves in the center tanks in the tank triplets I,II, V and VI. A complete segregation for the tank triplets I and II, IIIand IV, and V and VI is then obtained. The loading line 19 then willserve the forward segregation, the conduit 26 will serve theintermediate segregation, and the conduit 28 will serve the rearwardsegregation.

As appears from the above, all conduits and all valves with appurtenantdrive units are arranged in the central area, in connection with thecenter tanks. This gives a construction having relatively few pipes andvalves, and thus reduced building costs and easier maintenance.

On the vessel there is also arranged a separate emergency unloadingand/or stripping arrangement, as shown in FIGS. 8-10. This comprises aconduit 34 extending through all the center tanks 3 and being connectedwith separate outlets to each of the loading tanks 3, 5 and 6 viasuitable valves 35 and 36. In the pump room 15, the conduit 34 isconnected to double unloading pumps 37 leading to an external unloadingline (not depicted).

All the three bottom lines 19, 26, 28 is connected to the strippingsystem. For this purpose double valves are provided (marked with thesymbol "x2" in FIG. 5 in connection with the valves 20).

The main objective of the stripping arrangement is to drain all tankscompletely. In addition, it will be able to be used for "emergencyunloading" in the case of failure of bulkhead valves, and for transferfrom one tank to another. The system may also be used for individualfinal filling of the tanks. The pipe arrangement will be provided withdouble valves in order to satisfy the demand for segregation with twovalves.

In the following, the principal operations of the method according tothe invention will be further described.

The principal procedure is that all center tanks are firstly filled,either collectively or singly, whereafter the bulkhead valves to a wingtank pair are opened, and these wing tanks are filled by means of thestatic pressure difference. By means of this loading procedure therelatively large wing tanks are filled as quickly as possible, and thetime for the condition of standing waves in the tanks is reduced to aminimum.

As mentioned, a desired number, normally all, of the center tanks 3 arefilled as a first step. In case of bad weather, with wind and sea, thecenter tanks are filled one by one, the bulkhead valves 18 being keptshut. In weather conditions giving small or little movement of the ship,the center tanks may be filled simultaneously, the bulkhead valves 18between the center tanks being kept open.

When all the center tanks are full, all the bulkhead valves 18therebetween are opened, if they are not already open. The loading linevalves 20 to the center tanks 3 are shut, except to the center tank forthe wing tank pair 5, 6 which is to be filled firstly, for example intank triplet III or IV. The bulkhead valves 17 to these two wing tanksare opened, and the wing tanks then are filled as quickly as possible bymeans of the static pressure from the center tanks and the oilproduction from the loading line 19.

When the filling height of the wing tanks for standing waves has beenpassed, and the filling speed approaches the loading rate (i.e. the rateof flow of the oil production), the bulkhead valves 18 are closedbetween all the center tanks, whereas the loading line valve 20 to thetank triplet which is being filled, is kept open. The tank triplet thenis filled quite up to the desired level for transport.

Thereafter the bulkhead valves 17 to the wing tanks are closed, and theloading line valve 20 to the appurtenant center tank is closed, at thesame time as the loading line valves 20 to the next center tank to befilled, are opened.

The procedure is repeated when all the center tanks have been filled upagain.

By means of the double conduit arrangement 19 and 26, filling of a tanktriplet as an alternative may be carried out by means of the unloadingpumps 23, 30, 31. In this operation all the valves 20 on the conduit 19are closed, except for the valves at the front and rear edge of thecenter tank of the tank triplet, and the pipe valve 38 in the pipe orconduit 19 is closed, so that loading from the production will takeplace via the forward valve 20 and filling by means of the pumps willtake place via the rearward valve 20. All of the valves 27 are opened,except in the center tank of the tank triplet. By means of the pipe andvalve arrangement in the pump room, one, two or three of the unloadingpumps 23, 30, 31 may be used to suck/unload from the center tanks 3except the center tank of the tank triplet, and to fill the tank tripletquickly so that the period for standing waves in the tanks is minimized.

When unloading the ship, one starts with the center tanks, or with adesired center tank. The content of the wing tanks is passed to thecenter tank or center tanks. The flow to the sucking line becomeseffective in that the ship is trimmed somewhat afterward. By means ofthe shown pipe arrangement the operations can be carried out in a simpleand efficient manner.

We claim:
 1. A method of offshore oil loading of a tanker (1) comprisinga number of center tanks (3) arranged in the longitudinal direction ofthe vessel, and a number of wing tanks (5, 6) located in pairs on eitherside of respective center tanks, a loading line (19) being connected tothe center tanks (3) via respective values (20), and adjacent centertanks (3) being connected to each other via bulkhead valves (18),comprising the steps ofa) firstly effecting filling of a desired numberof the center tanks (3), these being substantially narrower and having acorrespondingly smaller volume than the wing tanks (5, 6), b) thereafteropening bulkhead valves (17) between a center tank (3) and anappurtenant wing tank pair (5, 6), and sluicing out oil from the centertanks (3) into the two wing tanks (5, 6) by means of the static pressurefrom the center tanks, the bulkhead valves (18) between the center tanks(3) being open, so that the wing tank pair (5, 6) is filled as quicklyas possible by suitable operation of the bulkhead valves (18) betweenthe center tanks (3) and of the valves (20) of the loading line (19) tothe center tanks (3), c) closing the bulkhead valves (17) to the twowing tanks (5,6) when these and the appurtenant center tank (3) arefilled to a desired level, and d) repeating the filling processaccording to the steps b) and c) successively for additional wing tankpairs (5, 6) after that all center tanks (3) have been filled again. 2.A method according to claim 1, when loading from an underwater buoyreceived in a submerged receiving space (9) in the bow portion of thevessel (1), wherein the oil is guided directly from the buoy to saidloading line (19), said line extending at bottom level through thecenter tanks (3) of the vessel.
 3. A method according to claim 1,wherein all the center tanks (3) are filled simultaneously during stepa), all the bulkhead valves (18) between the center tanks (3) being keptopen.
 4. A method according to claim 1, wherein the center tanks (3) arefilled one by one during step a), the bulkhead valves (18) being keptshut.
 5. A method according to claim 1, wherein the valves (20) of theloading line (19) to the center tanks (3), when these have been filledaccording to step a), thereafter are closed, except to the center tank(3) the wing tank pair (5, 6) of which is to be filled, whereafter thebulkhead valves (17) to the wing tank pair (5, 6) are opened, so thatthe wing tank pair (5, 6) is filled to a desired level, that thebulkhead valves (18) between all the center tanks (3) thereafter areclosed, whereas the valve (20) of the loading line (19) to said centertank (3) is kept open, so that the topical tank triplet (I, II, III, IV,V or VI) is filled quite up to the desired level for transport, and thatthe valve (20) of the loading line (19) to the center tank (3) is closedwhen the bulkhead valves (17) to the two wing tanks (5, 6) are closed,at the same time as the valve (20) of the loading line (19) to the nextcenter tank (3) to be filled, is opened.
 6. A vessel for offshore oilloading, comprising a number of center tanks (3) arranged in thelongitudinal direction of the vessel (1), and a number of wing tanks (5,6) located in pairs on either side of respective center tanks (3), aloading line (19) being connected to the center tanks via respectivevalves (20), and adjacent center tanks (3) being connected to each othervia bulkhead valves (18), wherein the center tanks (3) have a widthwhich is less than half the width of the wing tanks (5, 6), that alltank triplets (I-VI) consisting of a center tank (3) and an appurtenantwing tank pair (5, 6) are connected to each other via bulkhead valves(17), and that the loading line (19) extends through the center tanks(3), all valves (17, 18, 20) with appurtenant drive units being arrangedcentrally, in connection with the center tank area, a submergedreceiving space (9) being arranged in the bow portion of the vessel (1),for the reception of an adapted underwater buoy, the loading line (19)extending as a bottom line through the center tanks (3), the loadingline at its forward end being coupled to equipment (11) for directconnection to the buoy, and at its rearward end being coupled to pumpingequipment (23) for unloading purposes.
 7. A vessel according to claim 6,comprising a submerged receiving space (9) arranged in the bow portionof the vessel (1), for the reception of an adapted underwater buoy,CHARACTERIZED IN that the loading line (19) extends as a bottom linethrough the center tanks (3), the loading line at its forward end beingcoupled to equipment (11) for direct connection to the buoy, and at itsrearward end being coupled to pumping equipment (23) for unloadingpurposes.
 8. A vessel according to claim 6, wherein additional conduits(26, 28) are arranged in the center tank area, which conduits extendfrom pumping equipment (31, 32) at the rearward end of the vessel (1)and forwards to chosen tank groups, with a view to segregation of tankareas, double bulkhead valves (32, 33) being provided between theadjacent center tanks (3) between different segregation areas.
 9. Avessel according to claim 6, wherein the wing tanks (5, 6) have a tiltedbottom inward towards the center tanks (3).
 10. A vessel according toclaim 6, wherein the center tanks (3) are provided with transverselyextending baffle plates (4).
 11. A vessel according to any one of claims6, 8, 9 or 10, wherein an additional conduit (26) is arranged in thecenter tank area, which conduit extends from pumping equipment (23, 31,32) at the rearward end of the vessel (1) forwards to the forward tanktriplet (I) and is connected to each of the center tanks (3) viaseparate valves (27), and which conduit is arranged to cooperate withthe loading line (19) to fill the tank triplets successively by means ofthe pumping equipment.